TECHNOLOGY
point. We have old mines in Secunda,
Witbank and Middelburg which have to
be filled in with tens of thousands of cubic
metres of concrete of 10 to 15Mpa, up to
a maximum of 50Mpa concrete. We can
pump self-compacting or self-levelling
concrete into the mines to fill the cavities,
and for this we can quite efficiently use
recycled aggregates because it’s cheaper,
and in the case of fly ash it would be free. In
fact, some of the mines will pay you to take
it off their hands.”
The major costs associated with recycled
aggregate involve transport and labour. Cost
depends on how far it has to be transported
to a crusher. Thereafter, crushing is tedious,
says Du Plessis, because steel and other
substances such as PVC piping have to first
be removed and most of this process is done
manually at present. The mobile crushers
are also expensive, says Duvenhage. “There
are cranes which can remove the steel by
means of magnet, but that is even more
expensive,” says Du Plessis. Currently this is
affordable primarily for precast companies,
which build slabs to a high standard and
consequently have many rejects. Without
transport costs, these can be crushed and
put back though the system for reuse.
Amending SANS 1083
Duvenhage explains that one of the
problems with using recycled aggregate is
that there isn’t a SANS standard for it.
“There is a standard for natural aggregate
and crushed aggregate, called Aggregates
from Natural Resources – Aggregates for
Concrete (SANS 1083), which is in the
process of being changed. At the moment
there are several different specifications for
aggregate and the idea is to amalgamate
these into a single spec, still called SANS
1083. But the challenge for a recycled
aggregate spec is that you do not always
know what is in it. A pile of recycled
material might be anything from concrete
to glass to slag, and the requirements for
each of these is different. This is currently
being looked at by a team which has
experience in each type of material, with
the hope that out of their experience they
will start adding to this spec. The idea is
that this spec would give requirements
regardless of the source. There are currently
technical guidelines for the use of reclaimed
asphalt available from SABITA.”
“Sampling is the foundation of any
quality control. It can be tested on site
if the facilities exist, as most quarries
do. Otherwise, a sample has to be sent
to a SANAS [SA National Accreditation
System]-accredited laboratory,” she says.
Absorption of water is one of the
biggest criteria in concrete, because it has
to be consistent. “If water absorption is
going to be different every day, with one
batch requiring 200ℓ of water, the next
Aspasa's technical committee on quality
management, says recycling aggregate is not
done primarily for cost reasons, as recycling
concrete and aggregate is not a cheap
process. It requires manual separation so
as to get a uniform product through the
crusher. The crushing of recycled aggregates
is also harsh on crushers.
Costs aside, the challenge for the
recycling of aggregates is that a planned
development requires a standard
specification, whereas no engineer will spec
recycled aggregate for reinforced concrete
or strengths in excess of 20Mpa. Du Plessis
explains that the weakest link in cement is
the paste, or cement to water ratio, not the
aggregate.
“Now, if we introduce recycled aggregate
we could get a situation where the aggregate
is weaker than the cement paste and the
concrete is only as strong as the aggregate.
It would tend not to be used for reinforced
concrete structures, but only be used for
mass concrete; backfill; some foundations
for smaller buildings; non-structural
concrete; and backfill in piping trenches.”
Although such deployments could never
amount to large-scale use, Du Plessis points
out that what could be used on a massive
scale is secondary aggregate, which is
the ash dumps from the coal-fired power
stations. Most of the pulverised and burned
coal is dumped as piles of fly ash.
“That material has to be used at some
Recycling of aggregates requires
taking all types of materials.
26_QUARRY SA| NOVEMBER/DECEMBER 2018